1. Field of the Invention
The present invention relates to a lithium ion battery.
2. Background Art
Since 1980 or later, in particular, since the 1990s, with the development of cellular phones or laptop PCs, secondary batteries to be used as the electric power supplies for such devices have been required to be made higher in performances. Under such circumstances, lithium ion batteries (lithium ion secondary batteries), which have high energy densities, are mainly used as the secondary batteries in place of lead batteries or nickel-metal hydride batteries.
Recently, lithium ion batteries are higher in battery voltages and have higher output properties as compared to nickel-metal hydride batteries or lead batteries, and hence lithium ion batteries have attracted attention as the high output electric power supplies for electric automobiles or hybrid electric automobiles, or devices such as electric tools. For the purpose of applying lithium ion batteries to the applications requiring high output power such as electric automobiles or electric tools, it is required to attain larger current and more satisfactory output properties than in the application to the devices such as cellular phones. For example, in a (plug-in) hybrid automobile which at first travels by using only electric power of a battery and travels by using an engine in combination with the battery when the battery capacity comes to be lower than a certain value, a lithium ion battery which attains a high energy density while maintaining high output properties is required.
Examples of a method for increasing the energy density may include a method in which the capacity density of the material concerned is increased; however, the capacity of graphite mainly used as the negative electrode material for lithium ion batteries has already been improved in performances so as to be close to the theoretical capacity, and hence such a method as described above is not expected to provide any solution for increasing the energy density.
JP Patent Publication (Kokai) No. 9-204936 A (1997) discloses a method in which the energy density of a battery is increased by reducing the relative proportion of a current collector or a separator through increasing the thickness of each of the positive and negative electrode mixture layers. However, in JP Patent Publication (Kokai) No. 9-204936 A (1997), in each of the positive and negative electrodes, the thickness of the mixture layer is made to have a one-side thickness of 125 μm or more, and hence the thickness of the electrode is increased and consequently the properties at large current in applications requiring high output power are degraded.
Additionally, JP Patent Publication (Kokai) No. 2008-218248 A discloses a lithium ion battery in which the density of the negative electrode mixture is 1.4 g/cm3 or more and 1.7 g/cm3 or less and the coating amount of the negative electrode mixture on the both sides of the current collector is 6 mg/cm2 or more and 8 mg/cm2 or less. It is stated that under such conditions, the contact with the electrolyte in the active material interface is ensured and satisfactory output properties at large current can be ensured. However, in JP Patent Publication (Kokai) No. 2008-218248 A, the one-side thickness of the negative electrode mixture layer is approximately 28 μm at a maximum, and under such conditions, such a high energy density as required for plug-in hybrid electric automobiles cannot be attained.
In contrast to the above-described cases, JP Patent Publication (Kokai) No. 2008-103311 A discloses a method in which the electrolyte includes 4-fluoro-1,3-dioxolan-2-one and a satisfactory coating film is formed on the negative electrode, and consequently the 1 C cycle properties can be improved even when the one-side thickness of the negative electrode mixture layer comes to be as thick as 120 μm (the term 1 C means the current value at which the theoretical capacity can be completely discharged in an hour).